This invention relates to electrical distribution systems having conductor rails on which current collectors are in sliding contact and more specifically relates to an improved composite conductor rail for such electrical distribution systems.
A conductor rail provides a continuous contact surface from which to collect current by a sliding contactor consisting of a take-off shoe or slipper. Such conductor rails are used for electrical distribution in transit systems and overhead electrification systems for moving cranes and trolleys. Large currents may be demanded, e.g., in transit systems where the conductor rails, sometimes referred to as third-rails, run parallel to the wright bearing tracks and provide current to contactors moving at various speeds, the contactors taking off currents of 500 amps to two, three or four times that much depending on speed of transit vehicle and its weight load.
To transfer currents of this magnitude from the energy source to the moving vehicle, there must be minimal electrical resistance in the conductor rail. Further, the contact surface of conductor rails must be smooth and flat to allow a take-off shoe to make good electrical contact and also resist the wear of the constant sliding of the contactor under pressure. Accordingly, the conductor rail must be structurally sound to withstand this wear and pressure and at the same time be a good conductor having low electrical resistance. Low cost is also essential, especially for transit systems which may require miles of conductor rails.
Conductor bars of aluminum are presently used as the primary current carrier, having low electrical resistance, good structural strength and being lighter in weight and lower in cost than other materials previously used as the main conductors for electrical distribution, such as ferrous metals. Despite the advantages of aluminum conductor bars, problems of a relatively high coefficient of friction and lack of hardness to withstand the constant pressure and friction of the sliding collector still exist. A contact surface of harder material with good electrical conductance, such as steel, has been incorporated into composite conductor rails to withstand the wear from the constant friction of the sliding contactor.
Attachment of a contact surface or cap over the aluminum conductor bar has been accomplished in a variety of methods. One method shown in U.S. Pat. No. 3,341,669 utilizes a composite wherein the conductor bar has been bifurcated at its top surface and each side of this surface has been tapered inwardly with a cap of E-shaped cross-section placed over the conductor bar end and the outer legs of the "E" crimped over the conductor bar forming a mechanical interlock. In this arrangement the more ductile aluminum yields to the pressure of deforming the steel cap; and as the resilience of the steel causes the edge of the cap to move away from the conductor, the integrity of the composite could be adversely affected.
In another method of attachment shown in U.S. Pat. No. 3,733,696, a stainless steel cap is mechanically held by spaced nails driven through side legs of the cap and into an aluminum conductor bar. In a variation of that, pointed projections, sometimes called dinks, are punched into the cap side legs, forming dimples in the conductor bar that capture the projections. Conductive grease between the cap and the bar further aids good electrical connection. While these approaches have been very successful, some variations in the smoothness or levelness of the cap contact surface, and some variations in the conductive path are created by these spaced attachment methods.
Other arrangements have included drilling holes through the side walls of the steel cap and either into or through the aluminum conductor bar, attaching the caps by bolting the caps to the bar or welding plugs driven into the holes in the bar and cap. All of the foregoing arrangements necessitate penetration of the cap for assembly and thus require either added labor or extra power to penetrate the hard steel contact cap.
Another method of attachment shown in U.S. Pat. No. 3,917,039 is clamping a spring-like cap over the aluminum conductor. In such an arrangement the retention is reversible and could permit dislodging of the cap accidentally.
Another composite of a contact cap on a conductor bar comprises an aluminum bar having a V-shaped concave cross-sectional configuration and a V-shaped contact strip held to the surface of the conductor bar by integral aluminum flanges folded over the edges of the contact strip. Such an arrangement is illustrated in U.S. Pat. No. 3,830,989.
The present invention provides an improved composite conductor rail and a method of making same.